The Food and Drug Administration wants GlaxoSmithKline PLC to add the strongest form of safety warning about heart-attack risk to the label of its diabetes drug Avandia, according to people with knowledge of the matter, a move that would compound the commercial woes of the once-popular medication.

Agency officials are pushing for a "black box" warning, these people say. The new label is still being discussed with the company and its final form isn't yet clear. In high-profile safety matters, the agency tends to have strong leverage.

The new warning would be a blow to GlaxoSmithKline, which had said there isn't clear evidence Avandia is more dangerous than competitors. Avandia already carries a black-box warning about a different side effect -- heart failure -- but a heart-attack warning would be more serious. Avandia's main rival drug, Takeda Pharmaceutical Co.'s Actos, carries a heart-failure caution, but doesn't have one for heart-attack risk.